The present invention relates to the heating of digitally imageable lithographic printing plates and more particularly to the preheating of a plate in preparation for imaging or to the post heating of a plate after imaging.
Present thermal printing plate imaging technology requires the preheating of certain types of plates prior to imaging while post heating or baking is used with other types of plates after imaging. The preheating step is required for those plates requiring heating to activate the plate so that it is responsive to the laser imaging process. An example is a negative working plate where the preheat serves to form a Bronsted acid in the coating, thereby making the coating imageable by subsequent exposure with a thermal laser which insolubilizes the coating in those areas struck by the laser. Post baking is often used to further harden a coating for improved run length. This post baking process is very well known for positive-working plates with coatings containing phenolic resins. In both situations, the plate must be heated to a specific temperature for the process to work successfully.
The present technique for heating such plates uses convection ovens with the oven being set at some temperature well above the desired plate temperature. The expectation is that the dynamics of the heat transfer process can be controlled such that the plate achieves and is maintained at the desired temperature as it is conveyed through the oven. There are several drawbacks to this technique:
1. The heat transfer is non-uniform across the plate. As the plate travels through the oven, different areas of the plate are subjected to different degrees of heating depending on relative proximity to the heat source and airflow dynamics. The leading edge of the plate enters the oven at ambient temperature, whereas the trailing edge is preheated by conduction within the sheet prior to entering the oven. Obviously both edges have the same dwell within the oven.
2. There is the potential to overheat or to underheat the plate if any variable in the heating process changes. For example, if the initial plate temperature or ambient air temperature varies, the resultant temperature of the heated plate in the oven will give rise to a different end point temperature.
3. The oven set point needs to be varied for different size plates or different plate thicknesses. The mass that needs to be brought to the desired temperature will be different for 0.008xe2x80x3 and 0.012xe2x80x3 plates.
4. There are substantial heat losses to the room environment. This necessitates a larger investment in energy costs to maintain both the oven temperature and the room temperature.
When the plates are not brought to the correct temperature, performance problems arise. For example, for negative-working plates, if insufficient Bronsted acid is formed due to underheating, the plate will not respond properly to the imaging laser radiation. This may be localized due to non-uniform heating that can result in hot or cold spots on the plate. Conversely, if the preheat is too high, it may begin to convert the coating to an insolubilized state, thus causing toning in non-laser imaged areas.
For the positive-working plates with a post-baking process, underheating results in insufficient hardening of the coating. The coating will then suffer from premature wear, and the press run length will be shortened. The convection ovens are often run at settings near the annealing point of the aluminum. Hot spots or overheating may result in distortion of the aluminum sheet.
The object of the present invention is to provide apparatus and a method for heating a printing plate either before or after imaging whereby precise plate temperature control can be achieved. More particularly, a platen of sufficient mass and heat capacity is heated and maintained at the desired printing plate temperature. The printing plates, which are of low mass compared to the platen, are loaded onto and maintained in heat transfer contact with the platen for the period of time required to heat the printing plates to the temperature of the platen. The printing plates are then unloaded from the platen and processed according to the type of plate.